Torque sensors are used in a lot of applications in mechanical industry, such as measurement, control and optimization of machines, quality assurance during assembly, safety and reliability testing, and so on.
The torque sensors are typically shaft-mounted. The most common shaft-mounted torque sensors currently used in mechanical industry are based on the strain gauge method, which measures the shaft deformation by bonding strain gauges on the surface of the shaft at opposite angles to the axial line and measuring their resistance change due to the torque-induced compressive and tensile strain on the shaft. The performance of such torque sensor based on strain gauges is mainly limited by low signal-to-noise ratio due to the small gauge factor of the alloy used in strain gauge, high sensitivity to the shaft surface treatment and to the property of the bonding glue and its process of bonding the strain gauge to the treated surface, and high sensitivity to environmental factors such as package stress, temperature and humidity change.
Another type of shaft-mounted torque sensor is based on the magneto-elastic effect, which is also called Villari effect, where whole or parts of the shaft that is made of certain ferromagnetic alloys, such as NiCo, Galfenol and Terfenol-D, change their magnetic property due to the applied torque. The magnetic property change then changes the inductance of one or more stator-mounted sensing coils, so the measured inductance change indicate the shaft torque. The performance of such a torque sensor based on magneto-elastic effect is mainly limited by the obvious nonlinearity and hysteresis together with the saturation limit of the magneto-elastic effect in different ferromagnetic materials, and the high sensitivity to the temperature gradient on the shaft and to the ambient electro-magnetic interference.
U.S. Pat. No. 3,581,562 discloses a torque sensor using a change of pressure in a closed space for detecting the applied torque. The torque sensor has a hollow body and two shafts, which are movable within the body both for rotation and for axial movement. Each shaft is mounted with a spring, such that the spring force has to be overcome to rotate the shaft. The shafts are provided with guiding grooves extending obliquely to a longitudinal torque axis of the torque sensor at the outside surfaces of the shafts. The body is provided with pins, which engage with the grooves. When the torque sensor is subjected to a torque which rotates the shafts, the guiding grooves moves relative to the pins, causing the shafts to move axially towards each other while rotating. The torque sensor has liquid filled chamber between the ends of the shafts, and a gauge, which measures the decrease of volume of the chamber caused by the axial movements of the shafts. The volume decrease is proportional to the torque. This prior art torque sensor has a limited accuracy and resolution.